1. Field of the Invention
The present invention relates to a power transmission device used in industrial robots, particularly pendulm robots.
2. The Prior Art
In transmitting power in an industrial robot to its positionable parts, it is increasingly required that there be no play between the members transmitting the power, i.e., in order to achieve the highest possible positioning accuracy. There is further a desire to be able to readily and quickly to replace different components as required, thus reducing any down time which may possibly occur.
A customary power means in industrial robots is a so-called ball screw which is rotated by a reversible and programmable electric motor. The ball screw is provided with a nut means connected to a driven member, which may execute different types of motion in one and the same plane as the screw. In a known device, the driven member is a pivoting arm, one end of which is connected to the nut means with the aid of two links, mounted on pins on either side of the nut means at one end of the link, and on pins on the driven member at the other end of the link. The mountings are usually ball bearings. However, when screw is rotated in one direction, there is a small scissoring movement in these links, i.e., their axes no longer have exactly the same angular orientation in thier mutually parallel planes. When the screw is rotated in the opposite direction, the links change their mutually relative positions so that there is a small angle in the opposite direction. The nut means is turned a small angle to a corresponding extent in one or the other direction, i.e., there is a certain amount of play. Since the mutually angular movements of the links incur undesired play in the power transmission, which in turn results in decreased positioning accuracy, this play should be reduced to a minimum. The most obvious known measure is to increase the length of the bearings, since the play disappears when the bearings are sufficiently long. However, such long bearings are voluminous since they form long projecting details requiring free space for their movements.
Another known method is to guide the nut means along a slide in a path parallel to the screw so that the nut is prevented from turning relative to the screw. The object of the present invention is therefore to provide the same result with considerably simpler means, namely to prevent the links from scissoring relative each other and causing the nut to be turned relative to the screw.